Klaus

It happens right at the air-water-interface. The goal is to find the extension length that will lead to a minimal offset in how the line continues under water vs. in air. If you have no offset at all, then your lens renders exactly the same image above and below the water line. This happens only when the lens and the port are optimally arranged with respect to each other. I have probably sent people on the wrong track when I wrote about „stright lines“. Please excuse my lack of eloquence, I am not a native English speaker. Correctly stated, the observed effect here is the lateral offset of straight lines entering the warter. I have cropped the critical regions from the nice images provided by interceptor121 (thanks again) and annotated it accordingly - the test is essentially like aligning an inkjet print head (and this comes from the publication I cited, not my imagination). null The optical laws have been derived through simple experiments and vice-versa we can verify optical geometry with such tests. Of course everyone is welcome to suggest additional approaches to complement and/or replace it, I am not at all saying that this is a mandatory exercise. I also think that having the port in the ideal position by no means implies that it’s the right port for the lens in question. It may be possible to derive this from the rest of the images, but I have no idea how and that is explicitely not the topic of this post. I have said what I wanted (and by now more than once), so I will stop posting in this thread. Cheers, Klaus

That is exactly what I meant, and if the information is correct that the image on the LEFT is with a longer spacer then that fits the interpretation. The shorter extension leads to barrel distortion and that causes a displacement to the outside (because that distortion does not happen in air). I‘m not trying to give a lecture on optics but to point out a simple test that will indicate in which direction one should make a change (displacement to the outside > longer, displacement to the inside > shorter). If that is true, then it is quite likely that already the second configuration tested will be the best one available to a given person. Let‘s just leave it with this and see whether anyone finds it useful. That is the main point here. For me it already is, because I couldn‘t find the information for either the Samyang lens or the Athena port (lack of perseverance). But I now know that this will have some limitations that I can only solve at the expense of more vignetting, so I won‘t bother. GAS cured, thanks to your help!

Thanks for the clarification. So let’s sum this up: If the lines in the test shot continue with an offset to the outside of the image (i.e. the uw part is slightly enlarged), you need to increase the distance between lens and port. That‘s the opposite of my initial interpretation of the publication text. I suppose that if you go beyond the optimum, then the lines will be off-set to the inside. And even a 5mm difference from the optimal dome position is visible in this test. Whether 5 mm matter in practice is probably a different question and practical considerations, such as vignetting or plain availability of parts are also limiting the options. For example going back to my own shot I wold thus need to add an extension to avoid the offset, but there is already some vignetting so I won‘t do this. It‘s reassuring that theory and test come to the same conclusion (as they should). With the calculations you can get an answer BEFORE you have the parts in hand, provided all the required information is available. Cool!

Dome port physics aside, I have to say it is a nice thermometer… I like it.

Nice picture, thanks a lot! Which one has the longer extension- the one on the left or the one on the right? Sorry if I don’t get it when I should…

I think the easiest may be if you fix a test-object to the cold-shoe of your housing via a strobe-arm - I made a sketch in case this sounds cryptic. That way it may be fairly straightforward to get a small series of comparable shots with different length extensions, perhaps quicker that mounting it on a tripod again every time. In the article I found they say "misaligned front" when the uw part is enlarged (and vice-versa); I interpret this as the rays from the dome converge in front of the lens EP, but perhaps it is the lens that's in front of the convergence point? If you can do the test, I'd appreciate to know about your conclusion. I just don't have those toys (yet) to do it myself.. 😞 If you look at Phil Rudine's recent post on the 10 mm Laowa and dome-position test, there's a split with a floating pool thermometer and you can see that the uw-part is slightly enlarged. Perhaps that's exactly why he took this shot, after all it's quite intuitive - we want those lines form the thermometer (aka dive-boat) to continue perfectly straight into the water.

Yes, as I pointed out this will not help for assembling an order. I also think (but haven’t tested) that the effect kind of scales with the camera-object distance, hence when comparing configurations you probably need to get the camera into the same spot, using a tripod or some other stationary holding thing. Pool or plastic box does not influence the physics, but you need a calm water surface for the split. I think that as I progressively succumb to GAS, I may come back to this once in a while and play around with the available combinations. I can then maybe find the best solution available for me, not necessarily the ideal one (as mentioned in the first response). And while this test can keep me amused on a rainy winter weekend, it certainly won’t improve composition or diving skills - this is what’s currently limiting most of my shots. I just like the concept; it is charmingly simple and direct, test the port exactly to what it is intended for. In the publication this is only the first step, they then continue on with simulations and calculations to deal with the errors that REMAIN after the alignment. This is obviously of no relevance for me but apparently improves the photogrammetry.

There is apparently an experimental solution to solve the lens positioning problem within a dome-port. I found a publication (see below) from a team that calibrates photogrammetry cameras for deep-sea rovers at an oceanographic research institute. Perhaps it is already well-known in this community – but I figured I’d share it anyways. Note: This is not about choosing the right dome size, curvature etc. – just the lens position relative to the dome. The idea is quite straightforward: The goal of the dome-port is to avoid refraction by permitting the light to traverse the water-glass-air interfaces at a 90° angle – there should be no difference between air and water then. This is only possible when the lens is in the perfect position, hence our dilemma. If the position is not optimal, there will be refraction (grossly oversimplyfied: the dome will act a little bit like a flat port). Now the trick is to TAKE A SPLIT SHOT of a straight object that enters the water. If the lens is well positioned, there will be no refraction and the lines continue perfectly. Otherwise, the lines will be displaced and slightly enlarged or reduced in size. In the publication they had the camera on a screw-mount and could adjust its position (and hence the lens) „live“. For a housed „normal“ camera one would vary the port extensions until the best possible image is obtained. I tested this with my newly acquired (second-hand) Athena Fisheye dome on an Olympus Pen housing with the Rokinon/Samyang 7,5 mm fisheye. This is probably a sub-optimal setup because the dome was designed for the Panasonic 8mm lens which is longer than the Samyang. The image clearly shows that the lines are displaced a little bit. It’s best if you zoom in to the air-water-interface on the right, which is nice and „flat“. According to the publication, I should thus vary the lens position = spacer length. If I understood the publication right, then a displacement towards the outside (as in my test-shot) translates to shortening the spacer (which I can’t – there’s none) whereas a displacement to the inside indicates that a longer spacer is required. I don’t own a spacer and thus cannot try to aggravate the issue either for further demonstration. This seems like a simple test to perform once you have the lens, the dome and a collection of spacers at your disposal. It is unfortunately not helpful if you are trying to figure out what to order. But perhaps with the help of friends or at a workshop this can be of interest. Certainly my representation of the physics here is oversimplified, certainly there are many more things to consider - I am not claiming to be a specialist. But experiments don't need to be absolutely perfect as long as they are informative and useful. Publication: Mengkun She, Yifan Song, Jochen Mohrmann and Kevin Köser: Adjustment and Calibration of Dome Port Camera Systems for Underwater Vision (2019), part of Lecture Notes in Computer Science, https://doi.org/10.1007/978-3-030-33676-9_6 (you need a subscription for this, but try Google with Authors / Title before you pay…)

Thanks Tim, much appreciated.

I am totally new to all this, but just received my first dome yesterday (used). Since this is a 10 cm Minidome for a pen housing I was thinking of getting a cap-type cover for it. Essentially a plastic hood that does not touch the glass of the dome. How should that be used to prevent the staining marks?

I think you have created a nice resource collection here for anyone who wants to try sharks without baiting! I agree on the dose-effect and greenwashing risks, but I suppose we can start way before the bait-box with that. Hence I would not want to break into an ethical discussion because someone publishes a respectful (!) picture of a shark that has been baited; this may have been a minor contribution to the total impact of the journey. I throughly enjoyed a dive with big groupers on Madeira this summer, although they were clearly way more accustomed to divers than they should be. Whatever their routine may be, or have been in the past - it is for vacationers like me. And it was a cool photo opportunity! So I‘m not that much „better“, if at all.

I am afraid that there is no place left on this planet where human influence is NOT happening. Thus, we can only resort to finding the best compromise rather than dreaming about an ideal scenario. Eco-tourism is an oxymoron, but some ways of doing it are certainly better than others. And the mere fact of flying around the globe to reach a remote location is creating more of our problems - so the baiting may to be that much of an add-on. We will only protect (sort of) what we know, can experience and love, I think Sir David said that first. There’s no shortcut here: Even conservation efforts WILL leave a footprint. But less so than brutal exploitation. For the animals like sharks it has been said above: They must be worth more alive, or else they will be slaughtered and sold. This just does not work without baiting, and there is no point arguing with fundamental economic principles. You cannot run a shark expedition with negligible chances of seeing a shark. That said, I have not been able to really accept this and perhaps will never join such a trip. But that is personal choice and I do indeed find it slightly reassuring that baiting at least still works… Annotating the pictures is certainly a good option, even more if you include the name of responsible operators. The animals are NOT less fascinating when they have been baited, so there is no need to hide it.

I think it has a better emphasis on the eyes, to me it tells a more focused „story“. I prefer the darker version. With a snoot I guess you might get the „eyelashes“ above the eyes to pop a bit more out of the BG, but that must be daunting to position the beam just right…

Really nice image! Wouldn‘t the darkening be achievable in post processing? I assume that does not give the same result as a snoot, but it may be worth a try. And then you can „dose“ the level of darkness in different effect strengths. It will require a lot of clicking to make the mask around all the fine features, but maybe on a rainy day… at least the crab will stay put 😉

All right, here are the pics. You can see the slave sensor in front view, then the diffuser with the corresponding hole. I restricted it just a bit with a piece of silicon hose - holds the fibre optic plug just fine. But for using it without the diffuser, I would need to slave it off another strobe, I think that can be a bit dicey. Hence some kind of a holder would be great. I included 2 images with a centimeter scale, this is only for a rough idea in the dimensions. As said before, an existing design would be great. Cheers, Klaus

Sorry, the strobe is stored in a room where we are currently hosting visitors- will take a few more day yet until I can take a picture. I imagine this to be a sort of slip-on ring, like the wide end of a snoot, that has a small inner ring on the side where you can place the fiber optic cable. But I am really only asking if someone already HAS a design for such a thing. For a custom development I think I could ask some friends… and do some testing … Hence a tried-and-true design would be preferred. Klaus

I found a Sea&Sea YS-120 strobe and it works well by placing a fibre optic cable into the corresponding hole of the diffuser. I would like to use the strobe also without the diffuser - has anyone in this community designed a ring-like holder or whatever device that allows to hold a cable in front of the slave sensor on that strobe? I would very much appreciate obtaining (or posting in response) the corresponding file.

Quick question, maybe off-topic: How is the pilot light affected by the fly eye microprisms ? Is it weaker, less defined or is it similar between the two models?

I once read in a forum that this is/was a frequent issue with the shutter on the E-PL3. Apparently, a hearty whack on your knee (or two) could get those back into operation. Unfortunately I can't find that thread anymore. I never tried it myself, obviously this is not exactly the kind of treatment that the Olympus manual suggests and I would not try this as the new "power-on routine". I guess getting it repaired is the most reliable solution, but the "whacking" might save you when you're on a trip?

I think you can directly link this to the DIY section 😉

My favorite pic this year is rom a dive in a local quarry. My wife and son didn't want to join me for a dive there (as almost always) because "it's cold and there's only dead wood and no fish". I found a buddy through the social media site of a local dive center and we enojoyed a really cool dive. A storm had broken off some branches from the trees just a few days before and that made the scenery a bit "surreal" in places. I'm cheating a bit but just to add a picture for the "feeling" of that dive. The favorite pic is the last one I took on that day, just before we were getting out - a simple look along the edge. I like it because of the landscape and because I accidentally even had a very small pike in it (see if you can find it). Technically, the shot is a horribly underexposed mess, but I was astonished how much I could pull it up from the raw-file. I'm giving a second life to old photography gear for my attemps at underwater shooting. This is an Olympus E-PL5 in the PT-EP05L housing (fits with some customization), the 14-42 RII kit zoom and an Inon UWL-H100 wetlens without the dome (let me know if you have a dome to sell!)

I can already see that new full frame kit ….

I really like the solution with the readers that Ben presented - maybe not ideal for any situation but likely good enough for a lot. Certainly as a low-threshold entry into the aging photographer theme 🙂 I have two questions, Ben: 1) Did the add-on glasses ever get in the way with your regulator? I suppose this could happen when they slip down below the mask, but I simply cannot wrap my head around if that would cause a problem or not. 2) glass lenses lose a lot of their refractive power in water vs. air because water is much denser. I suppose this is similar for the plastic lenses In the readers, though some of those materials have a higher refractive index than glass. In any case, I assume that you are using stronger lenses than for reading above water, how much stronger would you suggest as a first try? Thanks, Klaus

First of all, I think this is a really nice shot and the pose of the octopus(?) is really cool. You probably tried cropping out just this guy in a vertical, maybe even b/w. Almost looks like (s)he’s ashamed of skinny-dipping… I don’t know a lot about UW photography, but I am not sure that more shadows necessarily translates into greater perceived depth. Your strobe is off-center already (towards the top left, shadows are a bit to the bottom right). I think the suggestion to go a bit lower yet is a good one, maybe it’s not that easy when freediving to try a couple of different angles. But as said above, definitely a great moment to capture!